If you aren't running on pavement low pressure will not have negative effects. It will however; improve traction, give the operator a softer ride, distrubuted the weight better across fragile surfaces like lawns, among other benefits. If you do the math on your rear tires surface area you'd be shocked to see how little you actually need to support your tractor (probably between 3-6 lbs minimum).
Front tires if you're doing FEL work are a different story, but rears I also use the tread touch as a gauge for correct pressure.